docs: def __prepare_histogram(h1, h2):
      """Convert the histograms to scipy.ndarrays if required."""
      h1 = h1 if scipy.ndarray == type(h1) else scipy.asarray(h1)
      h2 = h2 if scipy.ndarray == type(h2) else scipy.asarray(h2)
      if h1.shape != h2.shape or h1.size != h2.size:
          raise ValueError('h1 and h2 must be of same shape and size')
      return h1, h2